During processing of substrates, such as semiconductor substrates, vias are sometimes produced for electrical connections between layers in an electrical circuit. A via (sometimes referred to as a Vertical Interconnect Access) are typically small openings in an insulating layer which may be used as a conductive connection between layers on either side of the insulating layer. One type of via is a through-silicon via (TSV), which is a vertical electrical connection that passes through a silicon wafer or die, and may be used, e.g., to produce 3D integrated circuits or packages.
The physical characteristics of the vias produced during manufacturing can affect device performance and reliability. Physical characteristics of vias that may affect performance include depth and one or more critical dimension or shape of the via (and in particular the bottom surface of the via), which may be, e.g., circularity, ellipticity, bottom radius or radii (or equivalently diameter) curvature, x/y dimension, shape, sidewall angle, roughness, etc., sometimes generally referred to herein as bottom critical dimension (BCD). Thus, it is desirable to measure the physical characteristics of vias during the manufacture process.
One technique used to measure the physical characteristics of vias is optical metrology. However, the low signal-to-noise ratio of optical measurements from the via bottom makes detection and accurate measurement of the BCD and depth difficult, particularly for small vias, e.g., with diameters of approximate 5 μm or less and/or with high-aspect ratios (depth:diameter is approximately 15:1 or greater). As electronic devices continue to decrease in size, the diameter of vias continues to shrink, and the aspect ratios continue to increase, making accurate measurement of the physical characteristics of vias more difficult.